1. Field of the Invention
This invention relates to nuclear fuel bundles and more particularly to spacers used with the nuclear fuel bundles.
2. Brief Description of the Relevant Art
Nuclear reactors have fuel bundle arrays constructed of individual fuel bundles. These individual fuel bundles have a plurality of side-by-side fuel rods. The individual fuel bundles typically include a lower fuel rod supporting tie plate, an upper tie plate for maintaining the fuel rods in upstanding relation, and a fuel bundle channel for enclosing the fuel rods between the tie plates. Water flows into the fuel bundle through the lower tie plate, is confined to a flow path immediately about the fuel rods by the bundle enclosing channel, and flows out the top of the bundle in a steam water mixture.
In the assembly, transport, and operation of the fuel bundle, there is a tendency of the individual fuel rods to move from their vertical side-by-side disposition along the 160" length fuel bundle. When the fuel bundles are installed and operating within a boiling water nuclear reactor, such movement causes unwanted vibrations, interferes with the designed nuclear efficiency and is otherwise generally undesirable.
In order to prevent such movement of the fuel rods, five to eight spacers are utilized at selected elevations along the length of the fuel bundle. It is the function of such spacers to confine the fuel rods with respect to the channel as well as to maintain the designed side-by-side spacing of the fuel rods along the total length of the fuel bundle.
There have been developed over the years many kinds of spacers. While these spacers are desirable for controlling flow along the fuel rods, the spacers constitute neutron absorbing materials. This absorption of neutrons is detrimental to the nuclear economy and efficiency of the fuel within the fuel bundle. Consequently, it is always desired to maintain the least amount of neutron absorbing material in the spacers and yet have the spacers still function to perform their desired fuel rod separation function.
Spacers of the prior art have commonly been constructed of two materials. One material is the metal Inconel.RTM., a registered trademark of the International Nickel Company. The metal is an alloy of iron, chromium and nickel. This metal is strong, elastic and forms an ideal spring material. Unfortunately the metal has a relatively high neutron absorbing cross section.
The remaining material from which spacers are constructed is Zircaloy. Zircaloy is an alloy of zirconium with small amounts of iron, tin and other alloy metals. This metal has lower neutron absorbing cross sections but does not have the same strength of material properties and is unsuitable as a spring material. Consequently, most Zircaloy spacers have Inconel.RTM. springs incorporated to their structure.
The present invention is an improvement to a prior art spacer sometimes known as a "cross point spacer." This spacer has the majority of its structure fabricated from Zircaloy with Inconel.RTM. springs for the biasing of the fuel rods to the desired positions within the matrix of the spacer.
The cross point spacers are formed out of a lattice of vertically aligned tube members interconnected by strips. The tubes and strips fastened together form a continuous grid. The tube members typically form stops against which the fuel rods can be biased. This grid has individual spring members fastened to the tube interconnecting strips. The springs bias the fuel rods extending through the spacer onto tube stops formed integrally from the cylinders. The combination of the grid with its matrix of tube stops and strip fastened springs assures the uniform spacing of the fuel rods at selected elevations within a fuel bundle.
So-called cross point spacers have been proposed in a variety of configurations. In one configuration, the spatial relation or pitch between all of the fuel rods within the fuel bundle is maintained uniform. In another configuration, spacing or pitch between the individual fuel rods is varied. In either case, the improved method of spacer construction set forth herein is applicable.
It is also known to construct spacers with so-called swirl vanes incorporated to the spacers. In such construction, twisted pieces of material-twisted in the order of at least 90.degree., and more frequently in the order of 180.degree. to 360.degree., function to partially separate water and steam passing through the spacer. Specifically, water and steam rising through the swirl vane portion of the spacer have rotating momentum imparted. Water is thrown outwardly and onto the fuel rods where the more efficient generation of steam can occur. Steam within the rising steam water mixture maintains a more central flow path. For an example of a spacer incorporating such so-called swirl vanes, attention is invited to U.S. patent application Ser. No. 323,075, filed Mar. 14, 1989 entitled Swirl Vanes Integral with Spacer Grid, now U.S. Pat. No. 4,913,875, issued 4/3/90.